Realizing Excellent Infrared Nonlinear Optical Performance in Eu-Based Chalcogenides via Rational Cross Substitution Strategy

In recent years, rare-earth-based chalcogenides have gained attention promising materials in the field of infrared nonlinear optical (IR-NLO) applications owing to their exceptional physicochemical properties. However, they frequently encounter challenges such as adverse two-photon absorption and low laser-induced damage thresholds (LIDTs) caused by narrow optical band gaps (Eg), which limit their practical utility. In this study, we started with the centrosymmetric (CS) parent compound EuGa2S4to develop two new noncentrosymmetric (NCS) Eu-based chalcogenides, namely, EuZnSiS4and EuCdSiS4, employing a rational cross-substitution strategy. Despite having identical stoichiometry, both compounds crystallize in distinct NCS orthorhombic space groups [Fdd2 (no. 43) vs Ama2 (no. 40)], as confirmed by single-crystal structure analysis. Their crystal structures feature highly distorted tetrahedral motifs interconnected via corner-sharing, forming unique two-dimensional layers that host Eu2+cations. Furthermore, both compounds exhibit robust phase-matching second-harmonic generation (SHG) intensities of 1.5 × AgGaS2for EuZnSiS4and 2.8 × AgGaS2for EuCdSiS4under 2050 nm excitation. They also demonstrate high LIDTs (approximately 14–17 × AgGaS2), wide Eg(>2.5 eV), and transparency windows extending up to 18.2 μm. Particularly noteworthy, EuCdSiS4stands out as a pioneering example in the Eu-based IR-NLO system for successfully combining a broad Eg(>2.56 eV, equivalent to that of AgGaS2) with a significant SHG effect (>1.0 × AgGaS2) simultaneously. Structural analyses and theoretical insights underscore that the reasonable combination of asymmetric functional units plays a pivotal role in driving the CS-to-NCS structural transformation and enhancing the NLO and linear optical properties of these Eu-based chalcogenides. This study presents a promising chemical pathway for advancing rare-earth-based functional materials and suggests exciting opportunities for their future applications in IR-NLO technologies.